It is known, in particular in oil exploration, to determine the position of oil-bearing reservoirs on the basis of the results of geophysical measurements made from the surface or in drilling wells. These measurements typically involve sending a wave into the subsoil and measuring the various reflections of the wave off the sought-after geological structures—surfaces separating distinct materials, faults, etc. (technique of reflection seismic surveying). Other measurements are made from wells. Acoustic waves, gamma radiations or electrical signals are then sent into the subsoil. These techniques involve the processing of the measurements so as to reconstruct a model of the subsoil. The processing of the measurements is affected by uncertainties of various kinds. For reflection seismic surveying:
A first uncertainty is the uncertainty of picking. In geophysical measurements, picking refers to the tracking over the images or over the successive seismic sections provided by processing of the measurements of a seismic marker, for example a geological horizon, to define a surface. It is possible for the operator to confuse neighbouring markers or else to associate markers that are not part of one and the same surface. The picking uncertainty may be from a few meters to a few tens of meters; the value of a few meters corresponds more or less to the resolution, i.e. to the width of the pulses of the signals emitted into the subsoil; the value of a few tens of meters corresponds to a seismic marker positioning error.
Another uncertainty is the uncertainty regarding the velocity field in the subsoil. The depth-wise rendering of the seismic measurements performed in the time domain (this depth-wise rendering makes it possible to pass from a temporal representation to a spatial representation); the temporal representation is that provided by the measurement instruments—which typically measure the reflections of the wave emitted. The spatial representation is obtained on the basis of hypotheses regarding the velocity field in the subsoil. It assumes in fact hypotheses regarding the nature of the materials traversed and regarding the speed of propagation of the wave in these materials. Now, a velocity field hypotheses is affected by an uncertainty. This uncertainty regarding the velocity field may lead to positional uncertainties of the order of 50 meters.
Another uncertainty is the uncertainty due to migration. In this field, “migration” refers to the operation which makes it possible to correctly reposition the events in space. The uncertainty regarding migration is dependent on the geological context; it may be almost zero for tabular or quasi-tabular structures and reach hundreds of meters for complex structures, such as salt domes with vertical or quasi-vertical walls.
The presence of particular structures—faults for example—attenuating the reflection or causing diffractions of the emitted wave causes other uncertainties also.
By virtue of the measurements performed in wells it is possible to chart the limits of geological structures. The information thus obtained is also tainted by uncertainties; these uncertainties originate essentially from the measurements of position of the well.
Uncertainties of registration are due to the transferring of the limits of geological structures measured in the well to any seismic section intersecting the well; they originate in essence from resolution problems.
French Patent Application No. 2 797 329 describes a method of devising impact position probability maps for a well. This document contains a discussion of the uncertainties related to time and depth migrations and corresponding positional errors.
In view of these uncertainties, the processing of geophysical measurements may lead to the proposing of several realizations of the structures of the subsoil. For reservoirs—which are the structures sought by oil exploration—surfaces are proposed that are representative of the top of the reservoir, of the bottom of the reservoir and of interfaces between intermediate layers. A realization then consists of a possible set of surfaces representing the reservoir, corresponding to a set of hypotheses. Typically, in oil exploration, up to 300 realizations are provided, corresponding to the various possible hypotheses. By way of indication, the provision of a realization (so-called reference realization) on the basis of the results of geophysical measurements is an operation which, even with the assistance of a software tool, corresponds to considerable work, of possibly as much as 1 month/man for a practised specialist; when the seismic survey has been interpreted, the construction of a surface description of a reservoir may take a week or more. The collecting of the uncertainties also represents a week, their integration into a piece of software a day and the calculation of the realizations a few hours. Over a global area of 20 km2, a surface may be represented by triangles of side 100 m, hence a few thousand points per surface. A realization may involve 1 to 5 intermediate surfaces in addition to the surfaces of the top and of the bottom of the reservoir.
French Patent Application No. 2 652 180 describes a process for modelling surfaces. It suggests the use of a meshed computerized description of the surfaces. With some of the nodes of the meshed surface are associated constraints—representative for example of the known position of certain nodes or of a given uncertainty regarding the position of certain nodes. The computerized description of the surface is thereafter adapted to the constraints, using a method referred to as “Discrete Smooth Interpolation” (DSI).
Moreover, to represent the petrophysical properties of a reservoir, use is made of a grid or meshed volume model, as described in U.S. Pat. No. 4,821,164. The process described in this publication comprises steps of constructing a separate plane for each of the critical surfaces bounding the volume and of constructing layers of cells as a function of the stratigraphic structure. The meshed volume model thus comprises a number of planes greater than the number of critical surfaces. The implementation of a volume model such as this also involves appreciable work. A volume model may exhibit from 200,000 to a million points, over some thirty successive planes. The provision of a grid may represent from 1 to 2 months/man of work.
European Patent Application No. 0801364 (corresponding to U.S. Pat. No. 5,844,564) discloses a method for generating a three-dimensional meshed model of a geological structure. This method involves modelling the surfaces of the geological structure. Following this, the region of interest is split up after which each one of the macroblocks resulting from the splitting-up operation is meshed. The meshing involves interpolation and an iterative process of meshing relaxation for adaptation to the surfaces. This document, like U.S. Pat. No. 4,821,164 provides three-dimensional meshing of a given structure.